A horizontal injection molding system includes, as major elements, a mold clamping mechanism for clamping a mold and an injecting machine for injecting a resin material into the clamped mold.
Generally, a mold clamping mechanism is comprised of a stationary platen for supporting a stationary mold member of the mold, a pressure receiving platen disposed parallel to the stationary platen for supporting a mold clamping cylinder, a tie-bar extending between the pressure receiving platen and the stationary platen, and a movable platen capable of guided movement by the tie-bar and moved by the mold clamping cylinder for supporting a movable mold member of the mold.
Movement and mold clamping are performed continuously with respect to the movable platen by the mold clamping cylinder. In recent years, however, proposals have been made such that the movement is effected by a separate mold opening/closing actuator while the mold clamping is effected by the mold clamping cylinder, so as to shorten the required process or engineering time and to increase the productivity (see, e.g., JP 3881764 B—hereinafter referred to as “Patent Literature 1).
In the apparatus of Patent Literature 1, a pedestal is placed on a frame via a sliding plate. A mold clamping cylinder is fixed to the pedestal. The pedestal is moved horizontally by a mold opening/closing actuator. A movable platen is placed on the pedestal through the sliding plate. Top end of an operating rod extending from the mold clamping cylinder is connected to the movable platen.
In a mold-open state, the movable mold stays away from the fixed mold. For mold clamping, firstly, the mold opening/closing actuator is caused to retreat. This causes the pedestal to slide on the sliding plate toward a fixed platen. The mold clamping cylinder moves together with the pedestal whereby the movable platen moves together with the mold clamping cylinder. As a result, the movable mold comes into contact with the fixed mold.
In the meantime, the movable platen is guided by the tie-bar. Top ends of the tie-bar pass through the mold clamping cylinder to project therefrom and pass between half nuts.
Upon contact of the movable mold with the fixed mold, half nuts are brought to come close to each other so as to pinch the top ends of the tie-bar. This brings the tie-bar and the mold clamping cylinder into a unitary whole. Next, the mold clamping cylinder is advanced to cause the operational rod to move forward. This causes the movable mold to be pressed against the fixed mold through the movable platen to thereby complete the mold clamping.
By making the moving velocity of the mold opening/closing actuator higher than that of the mold clamping cylinder, it becomes possible to shorten the moving velocity of the movable mold. To achieve precise guiding of the movable platen by the tie-bar, an outer peripheral surface of the tie-bar is brought into sliding contact with an inner peripheral surface of a through-hole.
Since the movable platen moves to the stationary tie-bar highly frequently, one or both of the inner peripheral surface of the through-hole and the outer peripheral surface of the tie-bar becomes inevitably roughened. Such roughening will not occur uniformly but unevenly. Such uneven roughening will render the associated slide resistance unstable. Such unstable slide resistance presents a bar in a conventional injection molding process, as explained next. As shown in FIG. 8A, a movable mold 102 is clamped with a fixed mold 10 with a mold clamping force P1. In FIG. 8B, a molten resin material 103 is injected into a cavity. An injecting force P2 at this time is larger than the mold clamping force P1. As a result, up until both forces are balanced, the movable mold 102 opens a distance or gap, L1 relative to the fixed mold 101. Gas contained in the resin material 103 is discharged through the gap L1.
As the resin material 103 solidified and shrunk, the distance becomes L2 which is smaller than L1. The movable mold 102 advances a distance corresponding to (L1-L2). This provides a good-quality molded article free of nests and sink marks.
If the movement of the movable mold 102 becomes unstable at this time, the gap L1 becomes excessively small or excessively large. If the gap L1 becomes excessively small, degassing becomes insufficient. If the gap L1 becomes excessively large, the resin material 103 leaks together with the gas. Thus, the particular injection molding process shown in FIGS. 8A-8C is unsuited to the mold clamping mechanism of Patent Literature 1. Therefore, there is a demand for an arrangement that can move the movable mold 102 smoothly with a relatively small force (mold clamping force P1).
Consequently, so as to satisfy the recent demand for productivity increase, needed is such an arrangement in a mold clamping mechanism having a mold opening/closing actuator and a mold clamping cylinder which can move a movable mold smoothly after operation for a long period of time.